1. Field of the Invention
The present invention relates to a tunnel magnetoresistive (TMR) effect element which detects a signal field and indicates a resistance variation according to the magnetic field, a thin-film magnetic head provided with this TMR effect element, a head gimbal assembly (HGA) provided with this thin-film magnetic head and a magnetic disk drive apparatus provided with this HGA. The present invention further relates to an testing method and inspection apparatus, and manufacturing method for the TMR effect element.
2. Description of the Related Art
With the increasing capacity and decreasing size of a magnetic disk drive apparatus, higher sensitivity and higher output are required for a thin-film magnetic head. In response to this demand, a TMR effect that can be expected to have a rate of resistance change at least twice that of a giant magnetoresistive (GMR) effect currently being applied to reading of a signal field, is increasing attention. Actually, the thin-film magnetic head provided with the TMR effect element for reading is positively developed.
The TMR effect element has a structure in which a tunnel insulating layer which acts as an energy barrier for a tunnel effect is interposed between a pinned-magnetization-direction layer (pinned layer) whose magnetization direction is fixed and a free-magnetization-direction layer (free layer) whose magnetization direction is variable according to a magnetic field applied thereto. The existence of this tunnel insulating layer causes the TMR effect element to have a higher element resistance value than other magnetoresistive (MR) effect elements. Such a high element resistance generally causes the element resistance and stray capacitance to form a low pass filter circuit and causes the cutoff frequency of the element to decrease, preventing the higher frequency performance that is indispensable to the high recording density. Furthermore, the high element resistance also increases shot noise derived from random motion of electrons responsible for electric conductivity in the element. As a result, the SN ratio of the element output decreases.
On the other hand, a magnetic disk drive apparatus using a perpendicular magnetic recording system is being introduced in recent years to realize the higher recording density. The perpendicular magnetic recording system has a higher level of medium noise compared to a conventional longitudinal magnetic recording system, and therefore requirements for a lower noise ratio of the magnetic head is relatively alleviated. Therefore, the use of the TMR effect element in the perpendicular magnetic recording system for reading is quite promising in that the high output of the element can be utilized effectively. However, in order to realize the higher recording density, the reduction of the element resistance is becoming unavoidable for the perpendicular magnetic recording system to respond to requirements for the higher frequency and improve the SN ratio.
Furthermore, in the TMR effect element, a current flows through not only a metal but also a dielectric material of the tunnel insulating layer. The characteristic of the tunnel current flowing through this dielectric material causes the temperature coefficient of the element resistance to incline toward the negative value side. When a TMR effect element with a relatively large negative value of the temperature coefficient is used for the magnetic head, the output may become unstable due to a resistance variation caused by a variation in the ambient temperature as in the case of the GMR head with a positive value of the temperature coefficient as metal conductivity. Especially, thermal asperity due to the contact between the element and the medium is becoming a problem.
As a measure for these problems, Japanese Patent Publication No. 2004-185676 A discloses a thin-film magnetic head including a TMR effect element having a flat temperature gradient of the resistance value of the element (head). Furthermore, according to U.S. Pat. No. 6,452,204 and US Patent Publication No. 2002/0164828 A1, the resistance value of a tunnel junction MR effect film is lowered to 5×10−5 Ωcm2 or below to avoid thermal asperity. Furthermore, according to U.S. Pat. No. 6,812,039, oxidation of a metal layer during formation of a tunnel insulating layer is supported by UV light and its resistance value is lowered to less than 10 kΩμm2.
Furthermore, Japanese Patent Publication No. 2004-234755A discloses a thin-film magnetic head in which a resistor is connected in parallel to a TMR effect element, and a resistance value of the element, a resistance value of the resistor and a product RA of the resistance value of the element and cross-sectional area of the element are defined. Furthermore, Japanese Patent Publication No. 2002-217471 A discloses a ferromagnetic tunnel junction element provided with a current channel formed in parallel to a tunnel current channel and describes pinholes formed in a (tunnel) insulating layer as this current channel formed in parallel.
In the case of the above-described TMR effect element or the thin-film magnetic head having such an element, there is a problem that a considerable amount of popping noise occurs even when the resistance value of the element or the temperature coefficient of the resistance is limited to a predetermined range.
The reduction of the resistance of the TMR effect element has been required as described above. In this respect, as is also described in Japanese Patent Publication No. 2004-185676 A, for example, it is assumed that “the absolute resistance value of the element (head) reduces” as “the proportion of pinholes in the tunnel barrier layer increases.” As a result, it is further assumed to be possible to “achieve a reduction of noise.” Indeed, the resistance-dependent shot noise or the like decreases as the element resistance decreases.
On the other hand, however, the popping noise increases depending on the element, causing inconvenience such as errors in reproduction output depending on the degree of noise. That is, a considerable amount of popping noise occurs even with the elements described in the above described documents which conform to predetermined specifications about element resistance, and so it is very difficult to improve the SN ratio through the reduction of noise and prevent errors in reproduction output.
Here, as a popping noise occurring in a TMR effect element, only magnetic factors derived from a magnetization state in the TMR effect element such as Barkhausen noise are conventionally known. However, as described above, a popping noise whose causes and factors for determining intensity are actually not clarified occurs, and it is quite difficult to implement measures.